The goal of this research is to understand in detail the fundamental electronic fine structure of heme and hemoproteins. In this work we will use the technique of electron nuclear double resonance (ENDOR) to elucidate the structure at and near the metal in a number of hemes and hemoproteins, which will primarily be of the low spin ferric variety. ENDOR will serve as a probe for biologically significant changes, as an analytical tool, and as a means to discover the heme wavefunction. The ultimate outcome of the proposed work will be a more complete understanding of how hemes and hemoproteins function in their biological roles. The specific goals of this proposed research are: 1. To study the electronic structure at heme-linked nitrogens and protons by ENDOR in order to detect changes in this electronic structure brought about by conformational changes of the hemoglobin molecule. 2. To perform a complete orientation-dependent ENDOR study on single crystals of low spin ferric metmyoglobin derivative in order to obtain a complete picture of the heme electronic wavefunction. 3. To identify the origin of the copper signal which appears from cytochrome c oxidase. 4. To perform studies on heme model compounds to aid in assignment of ENDOR signals from proteins and to help in understanding the basis for electron changes seen in hemoproteins.